Dell UltraSharp 2208WFP, Service Manual

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Service Manual
11
fig.6
D850 is a rectifier in which there are 4 build-in diodes, inverting AC to DC.
C854 is used to smooth the wave from rectifier. U850 is a highly integrated PWM controller.
Typical start-up current for U850 is only 20 uA, When current flow through R8 4 1/R842/R843 gets to
Pin 3 of U850, with VDD hold-up capacitor C85 5 , U850 is enough for starting up.
When U850 begins to operate Pin8 of U850 will output square wave to drive Q850, then the main
current flow get to GND bypassing through T850, Q850. Because of the change of current flow, wires in
the other side of T850 will induct current. In the same time, the current inducted by wires which
connected T850 Pin 1 and Pin 3, with components of D852, R8
56
and C8
55
,will be supplied to U850 for
normal operating.
When the sense voltage across the sense resistor R859, reaches the threshold voltage around 0.85v,
the output GATE drive will be turned off. Every time when the output of power supply is shorted or over
loaded, the FB voltage will increase, the build-in PWM output will then be turned off. Both of two will
prevent the power supply from being overheated under over loading condition. The PWM duty cycle is
determined by this current sense signal and VFB, the feedback voltage, when the voltage on sense pin
reaches V-
(PWMcopm)
=(V
comp
-
2V
F
)/3, A switch cycle will be terminated immediately, Vcomp is internally
clamped to a variable voltage around 0. 85 V for output power limit
When Q850 are turned off, the main current flow will be consumed through D85 1 , C8 75 ,
R853/R854/R855 and D853 , This will prevent Q850 from being damaged under large current impulse and
voltage spike.
2.2.4) DC_2
4
V ,12Vand 5V Output Circuit and Feedback circuit: (fig.7)